Duct

ABSTRACT

The present invention relates to a duct for conducting a flowing primary fluid, having a thermally conductive wall with exterior ribs having an exterior side through which a secondary fluid can pass, at least partially, whereby the duct wall comprises a structured surface on its exterior side in addition to the ribs. The invention further relates to a heat exchanger with an inventive duct and an air condenser, particularly a natural-draught condenser, with inventive heat exchangers.

The present invention relates to a duct, a heat exchanger, and an air condenser according to the preambles of the independent claims 1, 14, and 17.

In a variety of applications, fluids, i.e. liquids, gasses, or mixtures thereof, such as water, steam, or air, are employed for transporting heat. To transfer heat, apparatuses otherwise known as heat exchangers are used. Heat exchangers usually have at least one duct through which a first fluid, hereinafter the primary fluid, is conducted. Heat is exchanged between the primary fluid and the environment across the wall of the duct. The environment can be a directly adjoining component, or a second fluid, hereinafter the secondary fluid for purposes of distinction.

In power plant technology, the primary fluid is usually hot water or steam and is conducted on the primary side of the heat exchanger, also known as the steam side, i.e. the interior side for prevention of mass loss. In contrast, the secondary fluid is usually air, which surrounds and circulates around the heat exchanger on its exterior side, the air side or secondary side. In this case, the wall of the duct, besides conducting the primary fluid, serves for the heat exchange between primary and secondary fluids.

Heat exchangers with species-related ducts are employed a variety of ways in the power plant field in order to extract residual energy from a primary fluid that has passed through a heat power process. For example, heat exchangers in air condensers are used for recovering the boiler water from the exhaust steam of turbines. After passing through the turbine, the steam condenses into water in the heat exchanger of the air condenser, and the water is fed back to the boiler. This closes the boiler water cycle.

The ability of heat to be transferred over a surface will be qualified by the heat transfer coefficient. The heat transfer coefficient alpha (α) specifies which amount of heat, measured with the energy unit Ws between a square meter of the surface of a component and the adjacent air, will be transferred when the temperature difference between the components surface and the air amounts to one Kelvin. Alpha is not a pure value of a substance (like heat conductivity, density, or viscosity), it is dependent on the substance properties of the fluid, the roughness of the wall, the temperature range and the flow relationship in proximity of the wall. The heat transfer coefficient is set on the primary side of a steam condenser with flowing steam at approximately 3000 W/(m² K), while a is approximately 50 to 100 times smaller on the secondary side to the air.

The known duct system for power plant heat exchangers usually comprise walls with smooth surfaces on the inside so that the primary fluid flowing through is presented with optimally little resistance. That way, it is possible to minimize the energy expenditure needed to keep the primary fluid in a flowing state. Such heat exchanger pipes typically comprise ribs on their exterior side that faces the secondary fluid, for purposes of enlarging the heat transfer surface area. These ribs are frequently strips of aluminum sheet a few centimeters high and only a few millimeters thick that are soldered onto the exterior of the duct as a base pipe. The base pipe typically consists of a pressure-resistant steel pipe surrounded by an aluminum layer at least on the outside.

The flow of the primary fluid or the secondary fluid in or around the flow duct can occur in a laminar or turbulent fashion, the flow state forming in dependence on the average flow rate, the duct cross-section, and the cinematic viscosity of the relevant fluid, among other factors. In this, a barrier emerges in the region of the duct walls. Due to this boundary layer, the heat exchange between the fluid and the duct wall occurs substantially only with the part of the flow near of the duct wall. A majority of the heat capacity of the fluids flowing by can therefore not be used or only ineffectively. In the case of a laminar flow, the thickness of this barrier is especially big.

It is therefore known at the technical level that so-called turbulators within the flow duct assembly parts or in the flow duct are to be provided. Turbulators are strong turbulence producing structures, almost like holes, ribs punched out or tags that provide for a better mixing of individual flow pieces with the produced turbulence. In this way a clearly improved utilization of the heat capacity of the fluid flowing by on the wall can be achieved. Nevertheless, this leads to the fact that the technical flow resistance is significantly increased. It is therefore a clearly larger effort to displace or stop the primary or secondary fluid in the flowing state.

The optimal performance of a heat exchanger depends, among other factors, on the heat transfer coefficient and the flow resistance in the heat exchanger duct. This leads to conflicting demands on the flow relations in the heat exchanger. On one hand, a largely laminary flow with optimally few deflections in the duct is desirable in order to minimize flow losses. On the other had, a turbulent flow can be desirable, because it makes possible a larger heat transfer coefficient and therefore an improved heat transfer.

The known heat exchangers only partly satisfy these demands. Heat exchangers which exhibit a small pressure loss owing to their substantially laminar flow or surge generally make possible only a small heat exchange, so that a majority of the heat energy of the passing primary or secondary fluids are emitted very slowly if at all. On the other hand, heat exchangers make possible an essentially good heat exchange with arranged inserts in the pipes or with turbulators arranged outside, but necessitate an introduction of the relevant fluids under high pressure so that through the inserts or turbulators the emerging pressure loss is equalized. It is usually necessary to provide means for increasing pressure, such as compressors, pumps, or suchlike.

The object of the present invention is thus to design a species-related duct for a heat exchanger which makes possible an improved heat exchange between the primary fluid flowing in the duct and the secondary fluid outside the duct, given a small pressure loss.

The object is inventively achieved by a duct according to claim 1, a heat exchanger according to claim 14, and an air condenser according to claim 17. Preferred developments are derived from the subclaims.

The present invention accordingly proposes a duct with a wall for conducting a flowing primary fluid, whereby heat is exchangeable between the primary fluid and the duct wall. The highly heat-conductive duct wall inventively is comprised of ribs with an additional structured surface on its exterior side facing the secondary fluid.

The heat exchange between the primary fluid and the duct wall thus occurs given a smooth internal wall. This brings a reduction of pressure losses in flow of the primary fluid compared to the known ducts, heat exchangers and air condensers. The associated initial relatively minor heat exchange with the primary fluid is inventively compensated by means of a heat exchange on the exterior of the ducts which is intensified, and even surpassed, by turbulences in the secondary fluid. The heat exchange between primary fluid and secondary fluid is increased as a result of the flow of the secondary fluid between the ribs being purposefully mixed more strongly by means of structurings that generate turbulence.

The structuring exhibits then by preference relatively weakly rounded shapes with only slightly sharp edges. Thereby a laminar flow of the flow duct in secondary fluid emerges only in local and limited micro turbulence in the area of the wall surface. The global flow of the secondary fluid then follows the further laminar while through the micro turbulence a clear reduction of the thickness of the barrier on the ducts outer wall is attained. This solution has the advantage that only the most minimal increase of the flow resistance happens on the secondary side, while the heat transfer coefficient is greatly increased there. In other words, the secondary flow is not so strongly disrupted that it fixes a large turbulence field or an entirely turbulent flow in secondary fluid.

The surprisingly beneficial effect of this configuration is felt particularly strongly in power plant heat exchangers or air condensers such as natural draught cooling towers, wherein air is typically employed as the secondary fluid with little internal friction compared with water. In the case of natural draught condensers and industrial heat exchangers, this is augmented by the quasi self-generated flow which the air generally exhibits with the aid of physical effects, as a result of which the surface structures for generating turbulence do not produce any notable impairment of the flow of the secondary fluid or require the enhanced propulsion capacities of pumps.

This micro turbulence inducing surface structures can be configured fully or partly located in the flow walls of the ribs of the flow ducts. Also the interaction of two different surfaces is advantageous. In a preferred design plan the structured surface exhibits a macrostructure and a microstructure. There also the ribs' plate also provides the microstructure and the macrostructure is then embossed and finally the ribs are brought to the base duct.

The microstructure has to do with a molding such as round or squared dents or buckling that extend either as bumps or indentations over the outside of the flow duct. This special shaping already has the known advantage of producing relatively low flow resistance but also at the same time brings about a good reduction of the barrier thickness in the secondary fluid. The height or depth of the outer formation is by measured from the unformed surface of the rib plate as approximately 0.05 mm to 0.15 mm.

The microstructure has to do with the preference of a oblongness that is stretched in the direction of the flow where the cross section is formed of soft waves or ripples. The height or the depths of the waves are measured as approximately 0.3 mm to 1.0 mm from the unformed surface of the rib plates.

The structure surface can then be built for example on the rib surface itself or also through a lamination. For example, the ribs can exhibit an embossing that the structure surface creates on both rib sides. The duct, the rib or the lamination exhibit thereby a sufficiently high friction coefficient to the secondary fluid in order to reach the required micro turbulence.

With the structured surface, a decrease of the thickness of the barrier through especially small and limited micro turbulence in the flow of the secondary fluid in the area around the duct can be achieved, which makes possible an increased heat transfer. A boundary layer in the region of the exterior wall can be reduced. At the same time, based on the unique position and shape, the turbulence can be minimized to such a degree that the pressure loss in the secondary fluid flow is not substantially increased.

In the first place, it is possible in this way to improve the conflicting demands on a heat exchange between flowing fluid and a duct wall. Based on the structured surface, it is now possible to increase the heat transfer capacity substantially given a slight pressure drop in the primary fluid

The properties of the duct wall and the nature of the structured surface are advantageously adapted to the fluid flowing through, so that an optimal effect can be achieved. For instance, a very fine surface structure can be provided for a high-viscosity fluid, whereas a rough structure can be provided for a low-viscosity fluid. The flow rate of the fluid, which can also affect the structured surface, must also be taken into account.

The structured surface can be provided partly on the side facing the fluid. It is also advantageous when it extends over the entire length and/or periphery of the duct. It is advantageously disposed at the locations that are particularly important for the heat transfer. Accordingly, the duct can comprise a smooth surface in a region that is provided merely for propelling the fluid, whereas an inventive surface is provided in a region of the provided heat exchange.

In a development of the present invention, it is suggested that the structured surface comprise formations, that is to say elevations. These formations are advantageously formed in the duct wall and protrude into the secondary fluid flow. They also enlarge the surface area of the duct wall. The size, number, and configuration of the formations relative to one another is selected so that the influence on the pressure drop in the secondary fluid flow is largely negligible. At the same time, the formations induce turbulences of the fluid flows in the region of the duct wall between the ribs. This surface can be produced inexpensively by known means.

It is further proposed that the structured surface also comprise depressions. With the depressions, as with the formations that rise into the flow, it is possible to increase the turbulence of the secondary fluid flow in the region of the duct wall. There is further enlargement of the micro turbulence of the surface on the exterior of the duct, which further enhances the heat exchange with the secondary fluid. Moreover, a duct with external depressions is very inexpensive to produce.

It is further proposed that formations and depressions be arranged in alternation in the flow direction of the secondary fluid. A particularly beneficial heat transfer can be achieved that way given a heavily stirred secondary stream. Particularly when the formations and depressions are arranged at intervals based on fluidic considerations, the heat transfer capacity can be substantially increased depending on the flow mechanics of the fluid.

It is further provided that the formations and/or depressions form a uniform pattern. Thus, for example, the formations and/or depressions can be disposed staggered in the flow direction. The shape of the formation and/or depression can also be adjusted in order to achieve an optimal heat exchange capacity. Thus, the shape can take the form of a spherical segment, conical segment, pyramid or suchlike.

In a development of the present invention, it is proposed that the deviation of the formation and/or depression from a center line of the duct wall surface facing the fluid equals a few tenths of a millimeter. An increase of the pressure drop can be further reduced that way.

It is further provided that the deviation of the formation and/or depression from the center line of the surface facing the fluid equals a few hundredths of a millimeter. An increase of the pressure drop can be reduced further. Different deviations from the center line and shapes of the formations or depressions can be combined.

In a preferred development of the invention, it is proposed that a heat-conductive, permeable meander structure that is oriented in the longitudinal direction of the duct is disposed in at least one of the ducts, which is in thermal communication with a neighboring cover plate at least partially at its reversal points. “Meander structure” means an optimally uniform corrugated steel tape that extends over the entire width and length of the duct. The troughs of the corrugations of the steel tape form contact lines at which the tape is soldered or glued to the base pipe of the duct. On the other side, the peaks of the corrugations form contact lines relative to the overlying coverplates. This results in a rectilinear flow path through the meander structure in the direction of secondary fluid flow, whereas the undulating rib strip winds back and forth evenly in meandering fashion from a side perspective. An enlargement of the heat transmitting surface can be advantageously achieved this way. Furthermore, the meander structure can also be provided with a structured surface, whereby the heat transmission capacity can be further increased. The thermal connection can be achieved by soldering, welding, gluing, or suchlike.

Further proposed by the invention is a heat exchanger with ducts that are passable by fluids which interact with one another thermally, whereby at least one inventive duct is provided.

The heat transmission capacity of the inventive heat exchanger can be advantageously increased this way without having to enlarge its structural shape and/or accept a higher pressure drop in the flow of the primary fluid. An existing device can thus be retrofitted with an inventive heat exchanger with a higher heat transmission capacity, with no additional space requirement. Besides this, a smaller structural shape of the heat exchanger can be achieved given the same heat capacity, in order to gain space in an existing device, for example.

Beyond this, the rigidity of the duct that is provided with such a surface, but also of the heat exchanger overall, can be increased by means of the structured surface. It can thus withstand an increased mechanical strain.

It is further proposed that the ducts at least partly form a plate-shaped duct configuration. An easily adaptable structural shape of the heat exchanger can be achieved by stacking plate-shaped duct configurations.

According to a further embodiment, the heat exchanger comprises a plurality of stacked plate-shaped duct configurations, whereby different fluids in alternation can pass through neighboring plate-shaped duct configurations. Thus, good adjustability based on stacking enables a high heat transfer capacity to be achieved from one fluid to another fluid passing through different duct configurations.

Further proposed according to the invention is an air condenser for condensing steam, particularly turbine steam of a power plant, whereby steam that is to be condensed can be conducted to the heat exchangers by way of a steam supply line and partitions, and whereby lines are provided for condensate removal and inert gas discharge, whereby the heat exchanger is an inventive heat exchanger with the above described advantages. Based on the correspondingly increased heat transfer capacity, the air condenser can have a smaller structural shape and can be produced more cost-effectively.

As described above, the preferred embodiment of the inventive air condenser is a natural draught condenser, since here the above-described benefits are particularly prominent.

The invention will now be described in detail in connection with exemplifying embodiments that are represented in the drawing. Substantially identical components are assigned identical reference characters. Shown are:

FIG. 1: a perspective view of a part of an inventive heat exchanger;

FIG. 2: a first embodiment of an inventive rib with a microstructured surface;

FIG. 3: a second embodiment of an inventive rib with a microstructured surface; and

FIG. 4: the section IV-IV, indicated in FIG. 2, of a third embodiment of an inventive rib with a macrostructured surface.

FIG. 5: the enlarged segment of the sectional representation indicated in FIG. 4, showing a fourth embodiment of the macrostructured and microstructured surface according to the invention.

FIG. 1 represents a part of an inventive heat exchanger 1 having an inventive duct 2 for conducting a primary fluid. The duct 2 comprises a wall 3 with a broad, flat base profile, which consists of two parallel plates 4 and 5 disposed at a distance from one another, which are connected laterally to semicircular pipe profiles 6 and 7. In this instance, the duct wall 3 of the duct 2 is made of a pressure-resistant, corrosion-resistant steel coated with aluminum on the outside.

Disposed on the flat sides 4, 5 of the duct wall 3 are two corrugated aluminum tapes 8 and 9, also referred to as cooling ribs, which form the external ribs of duct 2. Disposed on the external reversal points 10 of the two corrugated rib strips 8, 9 are two coverplates 11 and 12, respectively. These coverplates 11, 12 increase the rigidity of the rib strips 8 and 9, increase the heat exchanging surface of the duct 2, and make it possible to stack several ducts 2 on or next to one another easily. Thus, heat exchange plates 1 can be easily formed, and easily installed in cooling towers and dismantled therefrom in stacked bundles.

At their inner reversal points 13, the corrugated tapes or rib strips 8 and 9 are in thermal communication with the flat exterior sides 4, 5 of the thermally transmitting wall 3 of duct 2. In the present example, the thermal contact is realized as a glued connection using a high-temperature-resistant and thermally conductive glue. The contact can also be realized as a soldered or welded joint. The rib strips 8, 9 are made of aluminum, as are the coverplates 11, 12, whereby other materials that are good heat conductors can also be utilized.

The undulating and externally covered rib strips 8 and 9 result in a variety of flow lanes 14 on the base profile for the secondary fluid. In cross-section, the consecutive flow lanes 14 together with the rib strips 8, 9 that wind back and forth and the coverplates 11, 12 form two meander structures 15 and 16, respectively.

As a result of the alignment of the rib strips 8, 9 perpendicular to the flow direction of the primary fluid, the primary fluid and secondary fluid can pass through the duct 2 in crossflow. Steam passes through the duct 2 as the primary fluid in the interior, whereas air can pass through the exterior lanes 14 as the secondary fluid. The duct 2 with its wall 3, through which the primary fluid passes, and the flow lanes 14 are thus disposed at such a distance from each other that the two fluids cannot mix.

On its exterior, the wall 3 of duct 2 represented in FIG. 2 comprises a rib 8 having a structured surface 17. In this instance the structured surface 17 comprises a microstructure, shown greatly enlarged, whose bases are square- and triangular-shaped structures, whereby alternating formations 18 and depressions 19 form a uniform pattern in the direction of primary fluid flow. The formations 18 and depressions 19 are staggered and spaced apart.

The microstructured surfaces 17, shown greatly enlarged in FIG. 3, extend only on the lateral margin zones of the rib sheets 8 or 9. The structured surfaces 17 comprise truncated pyramids 20 of heat-resistant plastic with a rectangular base which are glued onto the ribs 8. In this embodiment, elevations 18 are provided in consecutive pairs in the flow direction, which are disposed a great distance from one another. In this example, the height of the elevations 18 relative to the surface of the rib sheet 8 is approximately 0.07 mm.

As is evident from the section through a third alternative embodiment of the structured surface 17 which is represented in FIG. 4, this structured surface can also comprise a macrostructure having round formations 18 and depressions 19 in the rib sheet 8. These shapes can be pressed into the rib sheet 8 by spherical embossing dies before the rib sheet is compressed into a ripple shape and fastened to duct wall 3. The height or depth of the formations 18 in the rib sheet 8 relative to the surface of the sheet is approximately 0.3 mm.

FIG. 5 shows the section represented by “A” from the sectional view of FIG. 4, greatly enlarged. In the fourth exemplary embodiment of the structured surface of the rib shown here, a microstructure 21 of pyramid-shaped nibs is provided on the undulating macrostructure 22. Thus, this is an overlapping configuration of microstructure and macrostructure, but is also an embodiment in which the placement of microstructure next to macrostructure is advantageous.

The exemplifying embodiments represented in the Figures serve for illustration of the invention only, and do not represent limitations of the invention. In particular, the shape of the depressions and their configuration in the duct can be varied. 

1. Duct (2) for conducting a flowing primary fluid, having a thermally conductive wall (3) with exterior ribs (8, 9) having an exterior side through which a secondary fluid can pass, at least partially, characterized in that the ribs (8, 9) have a structured surface (17), at least in part.
 2. Duct according to claim 1, characterized in that the ribs (8, 9) have a fully structured surface (17).
 3. Duct according to claim 1, characterized in that the structured surface (17) comprises formations (18).
 4. Duct according to claim 1, characterized in that the structured surface (17) comprises depressions (19).
 5. Duct according to claim 1, characterized in that the structured surface (17) is aligned with a flow direction of the secondary fluid such that formations (18) and depressions (19) are arranged in alternation in the flow direction.
 6. Duct according to claim 1, characterized in that the formations (18) and/or depressions (19) form a uniform pattern.
 7. Duct according to claim 1, characterized in that the structured surface (17) has a microstructure (21) and/or a macrostructure (22).
 8. Duct according to claim 1, characterized in that the size of the formation (18) and/or the depression (19) of the macrostructure (22) in the direction of the secondary fluid flow equals a few tenths of a millimeter, in particular 0.30 mm to 1.00 mm.
 9. Duct according to claim 1, characterized in that the size of the formation (18) and/or the depression (19) of the microstructure (21) in the direction of the secondary fluid flow equals a few hundredths of a millimeter, in particular 0.05 mm to 0.15 mm.
 10. Duct according to claim 1, characterized in that a plurality of ribs is formed by a corrugated rib strip (8, 9).
 11. Duct according to claim 1, characterized in that the corrugated rib strip (8, 9) extends in the longitudinal direction of the duct (2).
 12. Duct according to claim 1, characterized in that the corrugated rib strip (8, 9) is covered by a plate (11, 12) on its side facing away from the duct wall (3), and forms a meander structure (15, 16) through which the secondary fluid can pass.
 13. Duct according to claim 1, characterized in that the corrugated rib strip (8, 9) is soldered and/or glued at its turning points (10, 13) to the duct wall (3) and the plate (11, 12).
 14. Heat exchanger (1) having at least one duct (2) according to claim
 1. 15. Heat exchanger according to claim 14, characterized in that the heat exchanger comprises a duct (2) with a passable meander structure (15, 16) having exterior coverplates (11, 12) which form a stackable duct configuration.
 16. Heat exchanger according to claim 14, characterized in that the heat exchanger comprises a plurality of stacked plate-shaped duct configurations, whereby different fluids in alternation can pass through adjoining plate-shaped duct configurations.
 17. Air condenser for condensing steam, particularly turbine steam of a power plant, whereby steam that is to be condensed can be fed to heat exchangers (1) by way of a steam supply line and partitions, and whereby lines are provided for condensate removal and inert gas discharge, characterized in that at least one heat exchanger (1) is a heat exchanger according to one of the preceding claims 14 through
 16. 18. Air condenser according to claim 17, characterized in that the air condenser is a natural-draught condenser. 